Quick Field Analysis: How Much Force Does a Top Rope Fall Produce?

Background

Earlier this week Rock and Ice published an article involving forces in a TR fall (1). They claimed they were able to reach forces as high as 7kN in their analysis. I did some testing on TR falls a few years back (2), but lieu of the article, I decided to revisit the topic once more.

Setup and Testing Procedures
The objective in this test is quite simple—determine how much force a typical TR fall produces under typical conditions. I conducted three series of tests:

(1) 160 lbs climber takes three TR falls w/ 180 lbs belayer; no twists in the rope at the anchor.

(2) Same as series one, but with two twists in the rope at the anchor.

(3) 180 lbs climber takes three TR falls w/ 160 lbs belayer; no twists in the rope at the anchor.

All three series of tests have the following common properties:

- The route was a 5.10c sport climb with five bolts, it was approximately 45’ tall and it was dead-vertical.

- Each of the three series incorporated three falls—one about 1/4th of the way up the route, the next ½ the way up, and the last at about ¾ of the way up.

- The belayer used a locking belay device; the belayer was not anchored to anything; the belayer did not jump or pull in slack mid-fall to manipulate the catch style.

- The load cell was placed on the anchor so as to measure the peak force on the anchor.

- The rope used was a Maxim Pinnacle 9.5mm, which has the highest impact force rating of any dynamic rope on the market (10.3kN, 31% dynamic elongation, 5% static elongation). This simulates a worst-case scenario with regard to rope choice, unless the climber is TRing on static rope.

- The belayer always maintained 0.5 – 2 feet of slack in the rope. Never was the climber weighting the rope, nor was there a loop of slack hanging down at the belayer or climber.

Setup Photo


Equipment
Measurements were taken with a 3000lbf load cell with a conditioner scanning at 520 Hz. The load cell was properly calibrated, and its accuracy was verified against three other load cells.
Results

Series One – 160 lbs climber, 180 lbs belayer, no twists in rope at anchor
- Fall 1 - 525.7 lbf / 2.33 kN
- Fall 2 - 515.6 lbf / 2.29 kN
- Fall 3 - 465 lbf / 2.07 kN

Series Two—160 lbs climber, 180 lbs belayer, two twists in rope at anchor
- Fall 1 - 462.8 lbf / 2.06 kN
- Fall 2 - 421.6 lbf / 1.88 kN
- Fall 3 - 426.8 lbf / 1.90 kN

Series Three—180 lbs climber, 160 lbs belayer, no twists in rope at anchor
- Fall 1 - 555.6 lbf / 2.47 kN
- Fall 2 - 570.3 lbf / 2.54 kN
- Fall 3 - 600.1 lbf / 2.67 kN

In the spirit of trying to replicate the results R&I got, both my partner and I climbed to the top of the route, pulled out two huge handfuls of slack (about 6-7’), and jumped off to simulate an inattentive belayer. The results were:

160 lbs climber, 180 lbs belayer - 766.3 lbf / 3.41 kN
180 lbs climber, 160 lbs belayer - 728.7 lbf / 3.24 kN

For fun, here is a graph showing the 766.3 lbf / 3.41 mentioned immediately above:

TR Force

While I do not have a video of series one and two, I have a video of series three:

youtu.be/f_hRE9isHx4

Summary
Out of nine falls, six by a 160 lbs climber and three by a 180 lbs climber, the average impact force presented to the top anchor was 522 lbf / 2.32 kN. The maximum impact force presented in any of the nine falls analogous of a typical TR setup with an attentive belayer was 600 lbf / 2.67 kN with the 180 lbs climber and 526 lbf / 2.33 kN with the 160 lbs climber.

When the test was expanded to include TR falls with free-fall drops simulating a large amount of slack in the rope, the maximum impact force I recorded was still only 766 lbf with the 160 lbs climber and 729 lbf with the 180 lbs climber. Also, keep in mind this entire test set was done with the single hardest-catching dynamic rope currently manufactured.

Reference
(1) rockandice.com/lates-news/C…

(2) mountainproject.com/v/top-r…

I think if you reach 7kn your belayer was asleep...

were they using a tired gym rope on a much shorter route with an autolock device?

That's super interesting thanks for doing this!

Climbers are nerds, and I love it. Nice work. Seems like there might be some KN sandbagging going on at R&I.

Hmm...R&I device calibration issues?

Both tests had roughly the same fall factor (R&I 0.11, 20kN 0.14). The main difference was that R&I used an almost totally static belay with a 200lb climber, and 20 kN had a 160 lb climber and a 180 lb climber alternating falling and belaying. With these conditions, you would expect higher numbers from the R&I tests, but nearly double is surprising.

rgold wrote:Hmm...R&I device calibration issues? Both tests had roughly the same fall factor (R&I 0.11, 20kN 0.14). The main difference was that R&I used an almost totally static belay with a 200lb climber, and 20 kN had a 160 lb climber and a 180 lb climber alternating falling and belaying. With these conditions, you would expect higher numbers from the R&I tests, but nearly double is surprising.

I tried to track down the author to ask. The article says it was written by Tyler Stableford. I looked him up on Facebook. There is one Tyler Stableford on FB, he is listed as living in CO, and listed as working for R&I. However, when I asked him about it he said he did not write the article and does not work for R&I anymore, so who knows. I might try to email R&I directly and ask. It seems really unlikely anchoring the belayer down would double the impact force. On the smaller falls, I was barely even lifted off the ground. We are talking a foot, if that. Yet, even R&I's "standard" TR falls were just under double mine—3.5kN on average. One thought might be that they are using an analog dyno, which I know can produce inaccurate results on dynamic loads, but that's just a guess since they dident comment on their equipment usage.

Worth noting that the R&I article was originally posted online in 2013. They just recycled it, which they seem to do pretty regularly...

Your numbers seem much more reasonable than R&I.

I read somewhere that the maximum force someone was able to impact on the top piece of gear (not fall factor 2 and I don't remember the fall length) in a lead fall scenario with a human belayer was on the order of 9KN which makes sense given the max impact force of most ropes and that energy is absorbed by the belayer too.

Here is a link to Geir Hundel's experiments geir.com/mythbuster.html unfortunately the link the beal's experiment is broken. FYI, 1030 LBS is 4.3 KN

Nice study.
It would've been instructive to see falls on an anchor, that is belay device connected to static anchor.

climber pat wrote: Here is a link to Geir Hundel's experiments geir.com/mythbuster.html 

Latest archive: GEIR HUNDAL the climbing mythbusters

I think you're actually citing this, though?  The theoretical fall factor

you should try this with a static rope. That'd be an interesting comparison.

Petzl claims 5.5kn impact force for a FF0.3 for their semistatic 10mm. (Presumably 80Kg, but I didn’t dig into the details)
https://m.petzl.com/US/en/Sport/Ropes/CLUB-10-mm 

7Kn at the anchor (so around 4Kn at the climber) sounds reasonable for a top rope fall with a lot of slack on a semistatic, probably with the belay device anchored. 

It would be interesting to see some tests confirm that, but I hope no one’s regularly leaving a big loop of slack hanging while they’re top roping on a semistatic, because that seems like it would hurt.

Who climbs with a semi static rope? Maybe R&I tested with a steel cable. 

Some gyms and lots of “high ropes course”/summer camps/etc use semistatics for top ropes. As long as there isn’t an insane amount of slack in the system it’s not a safety concern on top rope— the FF0.3 impact force petzl tested would be difficult to achieve without decking.

ETA: It’s much less work to give a birthday belay with a semistatic, so if most of your clients aren’t regular climbers it makes sense, if everyone belaying is closely monitored/not a hooligan.

The only scenario where impact forces exceed what you’d see in a typical lead fall would be if a belayer just straight up stopped taking in slack when the climber was two thirds of the way up and the climber jumped from the top of the wall.

Eli W wrote:

Some gyms and lots of “high ropes course”/summer camps/etc use semistatics for top ropes. As long as there isn’t an insane amount of slack in the system it’s not a safety concern on top rope— the FF0.3 impact force petzl tested would be difficult to achieve without decking.

ETA: It’s much less work to give a birthday belay with a semistatic, so if most of your clients aren’t regular climbers it makes sense, if everyone belaying is closely monitored/not a hooligan.

The only scenario where impact forces exceed what you’d see in a typical lead fall would be if a belayer just straight up stopped taking in slack when the climber was two thirds of the way up and the climber jumped from the top of the wall.

I have led and fallen on the Petzl semi static ropes many times. Used to avoid rope stretching falls into talus. No top rope load on one could be a problem.

Yeah, I’m not signing up to test it, but my assumption is that even on lead, the belayer getting yanked up into the first bolt absorbs enough energy that a semistatic probably won’t kill ya even if it breaks a couple ribs.

OSHA says a peak force of 8.9Kn for 2ms is acceptable for fall arrest systems, which seems like it would be challenging, even with a semistatic, to achieve in a climbing context without belaying of an anchor.